1. Field
The invention relates to a method and apparatus for qualitative and quantitative chemical analysis and, more particularly, to a method and apparatus for increasing the sensitivity, speed of analysis, resolving power, and signal-to-noise ratio of elemental, isotopic, and molecular analysis by properly forming, focusing, and detecting ions used in such analysis.
2. Discussion of the Related Art
Time-of-flight mass spectrometry has had a checkered history since it became commercially available following the innovations of the Wiley-McLaren design in 1955. These instruments were used for a wide range of applications facilitated by its very fast scan capability and/or ease of access to the ion source. However, because they were seen as having both low resolution and low sensitivity, these instruments never became widely utilized in the major areas of mass spectrometry. The resurgence of interest in the time-of-flight technique since the mid-1980s has been due to the advent at the forefront of mass spectrometry development of such techniques as laser and plasma desorption, laser ionization, and surface analysis, which require a complete spectrum for each ionization event and also an extended mass range.
Time-of-flight mass spectrometers operate on the simple principle that ions of differing mass/charge (m/q) ratios, but equal kinetic energy, when projected into an electric-field-free region, will separate according to their m/q ratios. Since charge is nominally unity, the separation occurs as a function of mass. If the ions travel a fixed distance (l) to a detector, then the "time-of-flight" (TOF) is given by the expression: ##EQU1## when (z is the ion charge, e is the electronic charge, and V is the accelerating voltage). In this case of constant ion energy, the flight time (t) is proportional to the square root of the ion mass. The light ions reach the detector before the heavy ones. By measuring the flight time, t, of ions from the ion source to the detector, the ion mass is determined.
In comparison to sector-field mass spectrometers or quadrupole mass spectrometers, a time-of-flight mass spectrometer is advantageous in some applications in which: (1) an extended mass range is required; (2) ion production is pulsed or transient; and (3) the spectrum should be recorded in microseconds. Despite their advantages, the early time-of-flight mass spectrometers were not widely used. This was mainly a result of their poor mass resolving power which was particularly limited by (1) the length of the ion packet produced which depends upon the method of ion production and the geometric construction of the ion source, and (2) energy spread of an ion packet which is due to initial energy distribution and also to the method of ion acceleration. These two factors caused a certain time spread (.DELTA.t) at the ion detector, even for ions of identical mass-to-charge ratio, m/q.
A time-of-flight mass spectrometer has a high transmission efficiency, so its sensitivity should be very high. However, because a time-of-flight mass spectrometer works with short ion bunches which usually are only a few nanoseconds in length, the sensitivity of a time-of-flight mass spectrometer is degraded greatly when the ions to be analyzed are continuously produced.
Two important improvements were made in time-of-flight mass spectrometers as a result of the work of WILEY AND MCLAREN, REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 26, 1150 (1955), and MAMYRIN ET AL., SOVIET PHYSICS, J.E.P.T., Vol. 37, 45 (1973). In 1955, Wiley and McLaren described a time-of-flight mass spectrometer with an electron impact ion source using so-called "space focusing" and "time-lag" techniques. Following this technique, the time spread caused by the length of ion packets and by the initial energy distribution was substantially reduced. In 1973, Mamyrin et al. first used an ion reflector for the compensation of the energy spread of ion packets. By using "space focusing" and combining it with an ion reflector, higher mass resolving power was also attained, as published by KUTSCHER ET AL., with a transversely and longitudinally focusing time-of-flight mass spectrometer, in INTERNATIONAL JOURNAL OF MASS SPECTROMETRY ION PHYSICS, Vol. 103, 117-128 (1991). Owing to their higher sensitivity for pulsed ion production and the fact that no mass-range limit existed, time-of-flight mass spectrometers were favored for detecting large biomolecules produced by plasma desorption (MACFARLANE ET AL., BIOCHEM. BIOPHYS. RES. COMMISS., Vol. 60, p. 616 (1974)), secondary ion emission (BENNINGHOVEN ET AL., ORGANIC MASS SPECTROMETRY, Vol. 12, p. 593, (1977)) or laser desorption techniques (KARAS ET AL., INTERNATIONAL JOURNAL OF MASS SPECTROMETRY ION PROCEEDINGS, Vol. 92, p. 231 (1989)). A time-of-flight mass spectrometer for analysis of chemical species in a plasma jet was first studied by O'HALERON ET AL., TECHNICAL DOCUMENT REPORT NO. ASD TDR 62-644, PARTS I AND II (1964) (prepared under Contract No. AF 33 (657)-11018 by the Bendix Corporation, Research Laboratories Division, Southfield, Mich.). Some other instruments for sampling ions from atmospheric-pressure, however, were not built until the beginning of this decade (SIN ET AL., ANALYTICAL CHEMISTRY, Vol. 63, p. 2897 (1991); DODONOV ET AL., BOOK OF ABSTRACTS FROM THE TWELFTH INTERNATIONAL MASS SPECTROMETRY CONFERENCE, AMSTERDAM (1991); COLES ET AL., PROCEEDINGS OF THE FORTIETH ASMS CONFERENCE ON MASS SPECTROMETRY AND APPLIED TOPICS, p. 10 (1992); BOYLE ET AL., ANALYTICAL CHEMISTRY, Vol. 64, p. 2084 (1992); and MYERS AND HIEFTJE, MICROCHEMICAL JOURNAL, Vol. 48, p. 259 (1993).